1. Field of the Invention
This invention relates to the fabrication of integrated circuit devices and in particular to an improved system and method for aligning the masks used during formation of patterned photoresist on an integrated circuit.
2. Description of the Prior Art
One processing step used in the fabrication of integrated circuits is a photolithographic step, where a semiconductor wafer is coated with a light sensitive photoresist. Using a mask, the photoresist is exposed to light in a pattern in order to prepare the photoresist for etch processes. One method for projecting an appropriate pattern is the use of a projection system such as the Ultratech Stepper Model 900 projection stepper. One type of mask used during photolithographic steps is a chromed glass or quartz plate bearing the image to be projected onto the integrated circuit. Light is projected onto the mask and those areas on the mask which are not chromed transmit the light, with chromed areas of the mask preventing transmission of light. The transmitted light is projected by a complex system of prisms and lenses so that a clear and accurate image of the pattern on the mask is projected onto a portion of the photoresist layer formed on the semiconductor wafer.
A simplified diagram of a projection stepper such as the Ultratech Model 900 is shown in FIG. 1. Light source 8 provides white light which is filtered to remove ultraviolet light. Light from light source 8 passes through light tunnel 7. Light tunnel 7 limits the light which passes through light tunnel 7 to those light rays which lens 9 can focus. The light passing through light tunnel 7 is controlled by actinic shutter 5 and high-speed shutter 6. Actinic shutter 5 does not block light, as does a conventional shutter. Instead, actinic shutter 5 allows light of a selected wavelength and/or pattern to pass. In the Ultratech Model 900, actinic shutter 5 passes only light having wavelengths within the range of 450 to 600 nanometers, and is thus used to limit the bandwidth of the light used for aligning mask 1 with wafer 4. Actinic shutter 5 is not used during the actual exposure of photoresist and thus the light from light source 8 is not limited in bandwidth during exposure of the photoresist. The light from light source 8 is focused by lens 9 toward mask 1 and prism unit 2. Mask 1 carries the desired pattern for exposing the photoresist on wafer 4. The patterned light provided by mask 1 is directed by prism unit 2. The light from prism unit 2 is focused on the area wafer 4 containing photoresist which is to be exposed with the image of mask 1.
Before exposing wafer 4 to the image on mask 1, the image on mask 1 must be properly aligned with wafer 4, so that components of the integrated circuit formed using the patterned photoresist produced by this photolithographic step will be properly aligned on wafer 4. During the alignment stage, high speed shutter 6 is opened and actinic shutter 5 remains closed. Actinic shutter 5 passes only light having wavelengths within the range of 450 nanometers to 600 nanometers. The light used in the alignment process must not expose the photoresist. Thus the light allowed to pass by actinic shutter 5 is limited to wavelengths having insufficient energy to cause the photoresist to chemically react. Therefore, the light provided through actinic shutter 5 has no effect on the photoresist on wafer 4.
An alignment target is a relief pattern on wafer 4 in a specific selected pattern. One example of an alignment target is shown in FIG. 2. For example, this relief area can be formed by polycrystalline silicon etched into a selected pattern. Alignment target 15 is used to align mask 1 with wafer 4, thereby insuring that components of the integrated circuit formed using the photoresist exposed during this photolithographic step are properly aligned.
Actinic shutter 5 includes a pattern which corresponds in shape to the shape of alignment target 15. With actinic shutter 5 closed and high-speed shutter 6 open, the pattern borne by actinic shutter 5 is projected onto an image on mask 1 of alignment target 15. The pattern projected by light source 8 through actinic shutter 5 is slightly larger than the image of alignment target 15 on mask 1, therefore the light transmitted from mask 1 has a pattern corresponding to an outline of alignment target 15.
This outline of the alignment target is projected by prism unit 2 onto reflector 10. A samll aperture 17 is provided in the center of concave reflector 10. The outline of the alignment target is reflected off reflector 10, through prism unit 2, and onto wafer 4. A diagram excluding prism unit 2 and isolating reflector 10 and wafer 4 is shown in FIG. 3. Reflector 10 is designed so that light waves reflected from prism 2 onto reflector 10, and from prism 2 onto wafer 4 and thus reflected from wafer 4 back to prism unit 2 strike reflector 10 at a right angle. As shown in FIG. 4, light from reflector 10 which is reflected by a flat portion of the surface of wafer 4 is reflected at an angle equal to the angle of incidence .phi.. Therefore, light that is reflected from flat surfaces on wafer 4 does not enter aperture 17 (FIG. 1). However, light that strikes the edge of alignment target 15 as shown in FIG. 4 is scattered upon reflecting off the edge of alignment target 15. Some of the scattered light enters into aperture 17.
When the outline of the alignment target transmitted from mask 1 and reflected from reflector 10 is misaligned, for example as shown in FIG. 5a, very little of the light provided by the outline of the alignment target strikes the edges of alignment target 15. Therefore, very little light passes through aperture 17. Conversely, when the outline of the alignment target is properly aligned as shown in FIG. 5b, all of the edges of alignment target 15 are struck by light from the outline of the alignment target. Therefore, the light that passes through aperture 17 in reflector 10 is at peak intensity when the outline of the alignment target is properly aligned with alignment target 15.
Referring back to FIG. 1, the light that passes through aperture 17 is focused by lens 11 and passes through alignment target mask 12. A plan view of alignment target mask 12 is shown in FIG. 6. Alignment target mask 12 allows light to pass which conforms to the pattern of alignment target 15. The image projected through aperture 17 in reflector 10 is most intense in a pattern conforming to the shape of alignment target 15. Therefore, alignment target mask 12 confines the light passing through alignment target mask 12 to the pattern of alignment target 15. This helps insure that the light passing through the aperture of reflector 10 is reflecting from alignment target 15 and is not reflected from some other feature on wafer 4 which does not have a pattern conforming to alignment target 15. When the light reflected from reflector 10 strikes a pattern on wafer 4 which does not conform to alignment target 15, the light reflected from this pattern which passes through aperture 17 is an image which conforms to the other feature and not to the target. Alignment target mask 12 blocks enough of this light to prevent the erroneous reflected light from being perceived as a true alignment signal.
The light passing through alignment target mask 12 is focused by lens 13 onto photomultiplier tube 14. Photomultiplier tube 14 provides an electrical signal to alignment circuitry (not shown) proportional to the intensity of the light incident on photomultiplier tube 14. Thus, when the signal provided by photomultiplier tube 14 is at peak intensity, the image of alignment target 15 on mask 1 and alignment target 15 on wafer 4 are properly aligned.
Various phenomena can introduce error into the alignment system. A major source of error is standing waves of light which are generated in the photoresist on wafer 4. This phenomenon is particularly troublesome when the surface of wafer 4 is coated by unpatterned metalization which is covered by photoresist. When these standing waves occur, they cause spurious variations in the light intensity pattern of the alignment mask seen by photomultiplier tube 14.
FIG. 7a is a diagram depicting an ideal alignment signal as provided by photomultiplier tube 14 when mask 1 and wafer 4 are being aligned. The ideal alignment signal provides a clear peak when mask 1 and wafer 4 are properly aligned, as shown in FIG. 7a. FIG. 7b is a diagram depicting a typical alignment signal provided by photomultiplier tube 14 in FIG. 1 when photolithographic projection stepper 20 is aligning a wafer 4 which is coated by a metal layer covered with photoresist. The deviation of the signal in FIG. 7b from the ideal signal of FIG. 7a is caused by standing waves. The signal of FIG. 7b has not one peak as does the signal of FIG. 7a, but three peaks which leaves the computer of photolithographic projection stepper 20 guessing as to which peak, if any of the three, indicates proper alignment.
Others have attempted to control this phenomenon by limiting the light transmitted through actinic shutter 5 to frequencies of light which will not generate standing waves. For unknown reasons, this technique has proven to be ineffective.